1. Sergeev, I.N., Definition of Characteristic Frequencies of Linear Equation, Differ. Uravn., 2004, vol. 40, no. 11, p. 1573.
2. Sergeev, I.N., The Determination and Properties of Characteristic Frequencies of Linear Equations, Tr. Semin. I. G. Petrovskogo, 2006, vol. 25, pp. 249–294.
3. Sergeev, I.N., Properties of Characteristic Frequencies of Linear Equations of an Arbitrary Order, Tr. Semin. im. I.G. Petrovskogo, 2013, vol. 29, pp. 414–442.
4. Barabanov, E.A. and Voidelevich, A.S., Spectra of Upper Sergeev Frequencies of Zeros and Signs of Linear Differential Equations, Dokl. Nats. Akad. Nauk Belarusi, 2016, vol. 60, no. 1, pp. 24–31.
5. Bykov, V.V., On the Baire Classification of Sergeev Frequencies of Zeros and Roots of Solutions of Linear Differential Equations, Differ. Uravn., 2016, vol. 52, no. 4, pp. 419–425.